Long Term Evolution (LTE) is a mobile network standard for wireless communication of high-speed data. The standard is developed by the 3GPP (3rd Generation Partnership Project) and is specified in its Release 8 document series. In contrast to the circuit-switched (CS) model of cellular network standards, LTE has been designed to support only packet-switched (PS) services. Voice over LTE (VoLTE) is a mobile network standard to support voice calls over the LTE network. VoLTE delivers voice service as data flows within the LTE data bearer, and therefore avoids dependency on the present 2G or 3G CS cellular networks. However, while VoLTE technology is being introduced, and for a long time thereafter, there will be the need to support voice calls between VoLTE mobile devices and legacy CS network mobile devices.
One mechanism to provide this interoperability on a VoLTE mobile device may be to initiate voice calls on the VoLTE network, and to route the call to a VoIP-to-CS interface if the called mobile device does not support VoLTE. However, this method increases the call setup latency significantly and causes the VoLTE mobile device to experience degradation of service.